The present invention generally relates to the art of semiconductor fabrication. More particularly, the present invention relates to an improvement in the processing of integrated circuits which have resistors in a polysilicon ("poly") layer.
Resistors, particularly poly resistors, are used in many circuits in semiconductors developed on monocrystalline silicon substrates, especially static RAMS. Poly resistors can be formed by doping an entire layer of poly overlying a substrate with only a light concentration of dopant such as boron, and then, using a poly doping mask, doping remaining or other selected parts of the polysilicon layer more heavily with a dopant such as arsenic or phosphorous. One disadvantage, however, to this process is that it introduces boron in many areas of the device in which it is not useful or detrimental to the device.
Another disadvantage of this usual method of fabrication involves differences in etch rates of poly with more than one level of doping. After doping, in separate steps, the resistive areas and the desired conductive regions through masking processes, the subsequent masked etching (using a poly definition mask) removes the poly in a predetermined pattern to define desired poly lines, some of which will contain resistors. However, the poly regions which are only lightly doped will etch away at a slower rate than the more heavily doped regions of the poly, especially when a dry etch is used. Because the highly doped poly will be totally removed much sooner than the lightly doped poly, the underlying oxide or substrate near the conductive regions of the poly will be partially or totally consumed before the lightly doped regions of the poly are fully etched. If this oxide is consumed, the device will fail. Also, as more dense circuits are developed, the underlying oxide layer becomes thinner overall, thereby further aggravating this problem.
Another aspect of the background to this invention concerns the number of masks which are used. According to the usual process, after the poly line containing a resistor is doped and defined (requiring two masks--a poly doping mask and a poly definition mask), there will generally be implants to the substrate or other regions. This will require the resistors to be protected by a load implant mask (a third mask). Thus, this prior method requies at least three separate masks in fabricating semiconductor integrated circuit with polysilicon resistors.
The cost of fabrication of semiconductors is directly related to the number of discrete steps needed in a fabrication process. Each separate mask increases this cost. Moreover, each additional mask in a fabrication process reduces the yield per wafer, which is a vital concern.
Accordingly, it is an object of the present invention to provide a method for fabricating resistors in poly which would allow the resistor elements to be doped without introducing boron or other resistor dopants into other portions of the polysilicon layer.
Another object of the present invention is to provide a method of semiconductor fabrication wherein the lightly doped resistive portions and the highly doped conductive portions of a polysilicon layer may be removed without detrimentally consuming the underlying oxide or substrate.
Still a further object of the present invention is to provide a method for semiconductor fabrication including the definition of resistors which reduces the number of discrete masking steps.